As a major component for aircraft engine or industrial gas turbine, the compressor equipped with several rotational blades operates to draw in air and compress the same. Active researches have been done on the compressors, to find ways to maximize pressure ratio or efficiency at the respective ends.
The compressor often suffers performance deterioration of the entire system, due to unstable flow structure such as stall or surge occurring in the compressor when unexpected situation occurs near the operating line where high pressure and efficiency are available. For example, in the aircraft engine, when the pressure at the compressor outlet is instantaneously rises beyond appropriate range, airflow at the compressor inlet is separated from the blade surface and flow, entering “stall” state which causes instability of the engine or overall compression system.
To prevent stall, active control and passive control may be employed. One of the representative examples of the active control is to jet high pressure air to around the end of the blade where the compressor stall occurs first, thereby preventing compressor stall and extending the area of operation. For the passive control, a way of forming various forms of recesses such as grooves or slots and re-circulating high pressure air at the downstream to upstream through casing treatment, thereby preventing or delaying occurrence of stall and subsequently extending area of operation.
The former method is capable of preventing stall without compromising efficiency. However, the method is disadvantageous as it can be hardly adapted for use in high speed compressors. The latter method is easily adaptable to the compressors and can help improve operational stability in view of delay of stall occurrence. However, the method has shortcoming of negative influence on the performance of the compressor, such as deteriorated compressor efficiency in exchange for stall prevention.
In order to address shortcomings particularly related with the latter method, as illustrated in FIG. 12, KR Patent No. 10-1025867 proposes “Fluid stabilizer for axial-flow impeller”. As disclosed in KR Patent No. 10-1025867, the fluid stabilizer includes drive portions 700 arranged per regions divided along a circumferential direction of the casing 500, which are moved in a radial direction of the casing 500, thus causing an adjustment rib 620 to be inserted into an adjustment hole 610. More specifically, the casing 500 is divided into four regions, and the drive portions 700 are arranged in each one of the divided regions. This is applicable to a particular casing treatment structure with uniform arrangement of the adjustment holes in which the adjustment hole 610 is arranged in 12 o'clock position in the region I, the adjustment hole 610 is in 3 o'clock position in the region II, the adjustment hole 610 is arranged in 6 o'clock position in the region III, the adjustment hole 610 is in 9 o'clock position in the region IV. That is, the structure requires that the direction the drive portions 700 are moved, the directions the adjustment holes 610 are inclined, and the directions of the adjustment ribs 620 be in agreement with each other.
The above requirement causes a shortcoming of deteriorated effect of preventing or delaying occurrence of stall by re-circulating the high pressure air at the downstream to upstream, because, when it is assumed that the rotor is rotated in a clockwise direction, by the time the blade (i.e., rotor) passes the 12 o'clock position of the region I and about to exit the region I, the direction of slope of the adjustment hole 610 is opposite to the rotational direction l of the blade, according to which the flow does not enter the adjustment hole 610 or is reduced. Further, with reference to a radius of the casing, the slope of the adjustment hole 610 facing the blade edge varies according to the rotational position of the blade, thus giving negative influence on the overall stability of the axial-flow compressor.